Thermal Printhead

ABSTRACT

A thermal printhead (A) according to the present invention includes a heating resistor ( 5 ) formed on a substrate ( 1 ), an electrode ( 3 ) for energizing the heating resistor ( 5 ), and a protection film ( 6 ) for covering the heating resistor ( 5 ) and the electrode ( 3 ). The protection film ( 6 ) has a surface with a ten-point mean roughness of no smaller than 0.2 μm.

TECHNICAL FIELD

The present invention relates to a thermal printhead used as a componentof a thermal printer.

BACKGROUND ART

FIG. 6 illustrates a conventional thermal printhead (see patent document1, for example) used as a component of a thermal printer. Theillustrated thermal printhead B includes an insulating substrate 91 anda glaze layer 92 made of e.g. glass formed on the substrate. The glazelayer 92 is formed with an electrode 93 and a heating resistor 95. Theheating resistor 95 and the electrode 93 are covered by a protectionfilm 96 mainly containing glass material. A platen roller P is providedat a position facing the heating resistor 95.

In printing with the above thermal printhead, the platen roller Ppresses thermal recording paper S, which is an example of print mediums,onto the protection film 96, while the thermal recording paper S ismoved in the secondary scanning direction (right-left direction in FIG.6). Here, heat generated at the heating resistor 95 is transmitted tothe thermal recoding paper S through the protection film 96 fordeveloping color, in other words, printing.

In printing with a thermal printhead, so-called sticking may occur.Sticking is a phenomenon in which the thermal recording paper sticks tothe surface of the protection film and thus paper feed of the thermalrecording paper is disturbed. The sticking may result in defectiveprinting such as white streaks left on the thermal recording paper.

A method for preventing incidence of sticking may be to reduce frictionresistance due to the sliding contact between the thermal recordingpaper and the protection film. In the conventional thermal printheadshown in FIG. 6, the surface of the protection film 96 is formed to besmooth. Specifically, the protection film 96 has a ten-point meanroughness Rz (JIS B 0601) generally of no more than 0.1 μm. However,even if the surface of the protection film 96 is smooth, the stickingmay occur.

For reducing the friction resistance between thermal recording paper anda protection film, a thermal printhead may be arranged in a followingmanner. Specifically, in this thermal printhead, the protection film hastwo-layer structure including an insulating layer for covering theheating resistor and the electrode, and a conductive layer for coveringthe insulating layer (see patent document 2, for example). In this way,static electricity due to contact friction between the surface of theprotection film and the thermal recording paper can be efficientlydischarged by the conductive layer. Thus, it is possible to prevent thethermal recording paper from adhering to the surface of the protectionfilm due to the static electricity. Still, this arrangement of thermalprinthead cannot eliminate the incidence of sticking.

Another method for preventing incidence of sticking may be to reduce theforce for pressing the thermal recording paper onto the protection film.However, in this method, heat is not sufficiently transmitted to thethermal recording paper, thereby deteriorating the print quality.

Patent Document 1: JP-A-07-186429

Patent Document 2: JP-A-2001-47652

DISCLOSURE OF THE INVENTION

The present invention has been proposed under the above-describedcircumstances. It is therefore an object of the present invention toprovide a thermal printhead capable of preventing incidence of stickingand thus enhancing the print quality.

A thermal printhead according to the present invention comprises aheating resistor formed on a substrate, an electrode for energizing theheating resistor, and a protection film for covering the heatingresistor and the electrode. The protection film has a surface with aten-point mean roughness of no smaller than 0.2 μm.

Preferably, the protection film may comprise a first layer formed on theheating resistor and the electrode, and a second layer formed on thefirst layer.

Preferably, the second layer may be porous, and the first layer may benon-porous.

Preferably, the second layer may be conductive, and the first layer maybe electrically insulating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view schematically illustrating the principal portionsof an example of a thermal printhead according to the present invention.

FIG. 2 is a sectional view taken along the lines II-II of FIG. 1.

FIG. 3 is a micrograph showing a surface of a protection film of thethermal printhead according to the present invention.

FIG. 4 is a micrograph showing a surface of a protection film of aconventional thermal printhead.

FIG. 5 is a graph illustrating the relationship between the surfaceroughness of the protection film and the print length.

FIG. 6 is a sectional view illustrating the principal portions of theconventional thermal printhead.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention is specificallydescribed below with reference to the accompanying drawings.

FIGS. 1 and 2 illustrate an example of a thermal printhead according tothe present invention. The thermal printhead A according to the presentembodiment includes a substrate 1, a glaze layer 2, a common electrode3, a plurality of individual electrodes 4, a heating resistor 5, and aprotection film 6. In FIG. 1, the protection film 6 is omitted.

The substrate 1 is nonconductive and made of e.g. alumina ceramic. Theglaze layer 2 is formed by printing and baking glass paste to coversubstantially the whole of the upper surface of the substrate 1. Theglaze layer 2 serves as a heat storage layer. The glaze layer 2 has asmooth surface formed with the common electrode 3 and the individualelectrodes 4, and facilitates the bonding of the common electrode 3 andothers.

The common electrode 3 includes a plurality of branches 3 a extendinglike comb-teeth. An end portion of each of the individual electrodes 4is positioned between a pair of adjacent branches 3 a. The other endportion of each of the individual electrodes 4 is formed with a bondingpad 4 a. These bonding pads 4 a are electrically connected with outputpads of non-illustrated drive ICs. The common electrode 3 and theindividual electrodes 4 are formed by printing and baking gold resinatepaste, for example.

The heating resistor 5 is formed into a strip with a predetermined widthextending in a predetermined direction of the substrate 1, so as tobridge the branches 3 a and the individual electrodes 4. The heatingresistor 5 is formed by printing and baking ruthenium oxide paste, forexample. In the thermal printhead A, the individual electrodes 4 areselectively energized by the non-illustrated drive ICs, so that aplurality of regions 50 (one of them represented by cross-hatching, forexample) of the heating resistor 5 generate heat, serving as a heatingdot defined by a pair of adjacent branches 3 a.

The protection film 6 covers the surfaces of the common electrode 3, theindividual electrodes 4, and the heating resistor 5. As shown in FIG. 2,the protection film 6includes an electrically insulating first layer 6Aand a conductive second layer 6B. The first layer 6A is formed byprinting and baking glass paste containing SiO₂, B₂O₃, or PbO, forexample. The second layer 6B is a porous layer covering the first layer6A, and its surface has a ten-point mean roughness Rz of no smaller than0.2 μm.

The second layer 6B is formed in the following process, for example.

First, conductive glass paste is printed on the first layer 6A to form aconductive glass paste layer, which is to be baked at a temperaturelower than the softening point of the glass contained. The conductiveglass paste is a mixture of glass paste mainly containing SiO₂, ZnO, andCaO, and a resistor paste. The resistor paste is made by addingruthenium oxide grains with a grain size of 0.001-1 μm to a glasscontaining e.g. PbO, SiO₂, and B₂O₃. The amount of ruthenium oxidecontained in the conductive glass paste is 0.3-30 wt %.

It is favorable that the softening points of the glass paste and theresistor paste are higher than the softening point of the first layer 6A(the softening point of the above-described glass paste SiO₂—B₂O₃—PbO is680° C.). The softening point of the glass paste is 785° C., and thesoftening point of the resistor paste is 865° C.

The baking temperature of the conductive glass paste is 760° C. As thisbaking temperature (760° C.) is lower than the softening point of theglass paste and of the resistor paste, the glass component of theconductive glass paste layer does not flow, and ruthenium oxide grainsare surrounded by air bubbles which form air gaps. In this way, thesecond layer 6B is formed to be a porous layer. As the softening pointof the first layer 6A (680° C.) is lower than the baking point of thesecond layer 6B (760° C.), the first layer 6A is softened in baking thesecond layer 6B to be fixed to the second layer 6B intimately.

FIG. 3 is a micrograph, taken at 1500 magnifications, of the surface ofthe second layer 6B formed in the above-described process. As shown, thesecond layer 6B is a porous layer with a large number of air gaps. Theseair gaps are irregularly dispersed throughout the second layer 6B. Theshapes of the air gaps are also irregular. Thus, the surface of thesecond layer 6B is irregular as seen in a vertical section. Therefore,even if the second layer 6B is grinded for surface treatment, the secondlayer 6B has a ten-point mean roughness Rz of no smaller than 0.2 μm. Itshould be noted that the above-described process for forming the secondlayer 6B is only an example. When the baking temperature of theconductive glass paste or other conditions are changed, the size of theair gaps formed in the second layer 6B is changed, and thus the surfaceroughness of the second layer 6B is also changed.

FIG. 4 is a micrograph of the surface of the protection film of theconventional thermal printhead, taken at the same magnifications as themicrograph in FIG. 3. As shown, the surface of the protection film issmooth and has a ten-point mean roughness Rz of no greater than 0.1 μm.When printing is performed with such protection film, theabove-described sticking occurs, resulting in defective printing withe.g. white streaks. Through diligent research, the present inventor cameto focus on the relationship between the surface roughness of theprotection film and the incidence of sticking, and found that stickingcan be efficiently prevented when the ten-point mean roughness of theprotection film is no smaller than 0.2 μm. This indicates that, when thesurface of the protection film is formed rough, which is contrary to theconventional arrangement, the contacting area between thermal recordingpaper and the protection film is reduced, thereby reducing the incidenceof sticking.

Next, the above matter is specifically described below, based onexperiments performed by the present inventor.

A plurality of thermal printheads with protection films each havingdifferent surface roughness were prepared for printing on thermalrecording paper, and the printing quality was evaluated. Each of theprotection films of the thermal printheads had different surfaceroughness by changing the size of grains in ruthenium oxide, the bakingtemperature of the protection film, and other conditions. When thebaking temperature of the protection film was higher than the bakingtemperature of the conductive glass paste, the protection film wasformed to be non-porous, and a thermal printhead having a conventionalarrangement was made.

Conditions other than the protection film were the same in all examples.The experiment was performed utilizing thermal recording paper (modelnumber: 135LAB) of Ricoh Co., Ltd., under conditions with temperature of34° C. and humidity of 90%.

FIG. 5 is a graph showing the relationship between the surface roughnessof the protection film of the thermal printhead and “print length”.Here, “print length” represents the length of printed marks as seen inthe secondary scanning direction of the printhead, printed on thethermal recording paper based on predetermined printing data. If thesticking occurs, paper feed of the thermal recording paper istemporarily stopped, and thus the print length of printed data becomesshorter than that of the same data printed without sticking. Thus, bychecking the print length, the incidence of sticking can be examined.

As shown in the graph, when the protection film had a surface roughnessRz smaller than 0.2 μm, the print length was remarkably shorter thanwhen the surface roughness was no smaller than 0.2 μm. It can be seenfrom this that when the surface roughness Rz is no smaller than 0.2 μm,the sticking can be prevented and thus the print quality is enhanced.Further, when performing an experiment utilizing other variouscommercially available thermal recording paper, the sticking wasprevented with a surface roughness Rz of no smaller than 0.2 μm, and aresult similar to the experiment utilizing the above-described thermalrecording paper was obtained.

As described above, the protection film 6 has two-layer structureincluding the first layer 6A as a lower layer and the second layer 6B asan upper layer laminated on the former. The first layer 6A properlyachieves the expected function as a protection film such as insulationand water resistance for the common electrode 3, the individualelectrodes 4, and the heating resistor 5. The second layer 6B is formedas a porous layer as described above, and thus the surface of the secondlayer 6B is formed to have an appropriate surface roughness larger thana predetermined value.

As the second layer 6B is porous and thus has a surface roughness largerthan a predetermined value, even when the layer is worn in contact withthe thermal contacting paper, its function for preventing the stickingcan be properly maintained. Further, as the first layer 6A iselectrically insulating and the second layer 6B is conductive, noelectrification due to friction between the second layer 6B and thethermal recording paper occurs, thereby preventing trouble in feedingthe thermal recording paper that would otherwise be caused by theelectrification.

In addition to the above-described experiment, the present inventorfurther performed an experiment utilizing a thermal printhead providedwith a single-layered insulating protection film containing inorganicoxide. In this experiment, the surface roughness of the protection filmwas varied by changing conditions such as the additive rate of theinorganic oxide, and the baking temperature of the protection film. Thisexperiment showed that, even if the protection layer has only one layer,the sticking can be prevented when the layer has a surface roughness Rzof no smaller than 0.2 μm, similarly to the above-described protectionfilm having two layers. As seen from this, even with a single-layeredprotection film, it is possible to efficiently prevent the sticking bysimply forming the protection film to have an appropriate surfaceroughness larger than a predetermined size. Further, there is no need toreduce the force to press the thermal recording paper onto theprotection film for prevention of the sticking, thereby enhancing theprint quality.

The present invention is not limited to the above-described embodiments.Specific structures of the thermal printhead according to the presentinvention may be variously modified within the spirit and scope of theinvention.

For example, the surface of the protection film is not necessarilyporous. Further, the protection film does not necessarily have thetwo-layer structure including the insulating layer and the conductivelayer, but may have only one insulating layer. In other words, if theprotection film has a surface roughness of no smaller than 0.2 μm, itmay adopt any lamination state or components.

It is favorable that the present invention is used for printing onthermal recording paper, but may also be used to print on non-thermalrecording paper utilizing a thermal ink ribbon.

The thermal printhead according to the present invention is not limitedto have a flat glaze layer, but may have a glaze layer with projections.The thermal printhead may be of a thin-film type or a thick-film type.

1. A thermal printhead comprising: a heating resistor formed on a substrate, an electrode for energizing the heating resistor, and a protection film for covering the heating resistor and the electrode; wherein the protection film has a surface with a ten-point mean roughness of no smaller than 0.2 μm.
 2. The thermal printhead according to claim 1, wherein the protection film comprises a first layer formed on the heating resistor and the electrode, and a second layer formed on the first layer.
 3. The thermal printhead according to claim 2, wherein the second layer is porous, and the first layer is non-porous.
 4. The thermal printhead according to claim 2, wherein the second layer is conductive, and the first layer is electrically insulating. 